Construction union Unite secured a commitment from the joint venture after its labour only supplier, Bowercross Construction, was exposed for failing to pay around 200 workers on enabling works the correct overtime rates and deprived them of correct holiday entitlement.

Unite said this meant that workers were on average being underpaid by over £100 a week. The union estimates the final back-pay bill could be over £1m.

The issue came to a head last week when Unite revealed that it had obtained multiple payslips from workers that proved they were not being paid in accordance with the project framework agreement,  signed by unions and HS2 in April 2016.

A Costain Skanska joint venture spokesperson said: “Costain Skanska JV  uses approved suppliers to provide labour to some of our sites on the HS2 enabling works programme.

“The workers in question are paid via PAYE and receive full employment rights, which include a minimum of the London Living Wage, EU holiday and sick pay and welfare benefits.

“We have looked into Unite’s report concerning the industry agreement rate for overtime and weekend duty, as well as holiday entitlement. The company concerned has been instructed to remedy this with immediate effect so that we meet our required standards.”

It added: “CSJV remains keen to maintain a working dialogue with Unite to ensure that its members’ views are considered and any concerns can be addressed swiftly.

“We have offered Unite access to site inductions where they can recruit members, as well as a suitable meeting room for recruitment and member engagement purposes, so that people working on our sites can choose how to spend their break times.”

It is the second time unions and BCL and CSJV have clashed.

Last autumn Unite discovered that contrary to the agreement, BCL was not paying £1.49 a week per worker to provide the correct accident and death benefit cover.

When the CSJV was challenged on this matter it not only assured Unite that the issue had been resolved but also confirmed they had conducted an extensive audit costing £60,000 of its subcontractors and all other matters were in order.

Unite national officer Jerry Swain, said: “CSJV had no option once the scandal of the underpayments was revealed by Unite to immediately ensure the correct rates are paid.

“Questions remain about who knew and for how long; workers were being underpaid and exploited.

“The commitment to not only pay the correct rates but also ensures that everyone who has been underpaid will receive the correct back pay, demonstrated the value of unions.”